1. Technical Field
The present invention relates to a surface light source device, a liquid crystal display device, and a mobile device. Particularly, the present invention relates to an area light source device that can reduce light leakage in an effective emission region to enhance emission quality.
2. Related Art
FIG. 1 illustrates a section of a conventional area light source device. In an area light source device 11, a light guide plate 13 is accommodated in a frame 12, and a light source 15 is disposed while facing an end face (a light incident end face 14) of the light guide plate 13. In the light guide plate 13, a light introduction part 16 is continuously molded at an end of the light guide plate body 17. The light introduction part 16 includes an inclined surface 16a, and has a thickness greater than that of the light guide plate body 17. The light guide plate body 17 occupies most part of an area of the light guide plate 13. The light source 15 is mounted on a lower surface of a flexible printed board 18. The flexible printed board 18 is bonded to upper surfaces of the frame 12 and light introduction part 16 using an adhesive 19, thereby fixing the light source 15 to a position facing the light incident end face 14.
Three optical sheets, namely, a diffusion sheet 20 and two prism sheets 21 and 22 are stacked on the upper surface of the light guide plate body 17. A rim sheet 23 (a light shielding plate) is bonded onto the flexible printed board 18, the frame 12, and the prism sheet 22, and an effective emission region of the light guide plate body 17 is exposed from an aperture window 24 of the rim sheet 23. The effective emission region of the light guide plate 13 means a region where the light used as the light of the area light source device 11 is output, and a region where the light with which the liquid crystal display panel is irradiated is output (usually a region except the effective emission region is covered with a rim sheet such that the light is not output from the area light source device). A reflecting sheet 25 is provided on the lower surface of the light guide plate 13.
In the area light source device 11, the light emitted from the light source 15 is incident to the light introduction part 16 through the light incident end face 14 located at the substantially same level as the light source 15. The light incident to the light introduction part 16 is reflected by the lower surface of the light introduction part 16 and the inclined surface 16a, thereby introducing the light to the thin light guide plate body 17. Therefore, in the area light source device 11, the light of the light introduction part 16 is efficiently incident to the light guide plate 13 to irradiate the effective emission region of the light guide plate 13 with high luminance, and a low profile of the area light source device 11 can be achieved.
However, in the area light source device 11 having the structure in FIG. 1, the light leaks easily from the aperture window 24 of the rim sheet 23 through three main routes. First leakage light is light L1 that leaks directly from the light source 15. That is, the light L1 is not incident to the light guide plate 13, but incident to an end face of an optical sheet through the transparent adhesive 19 that bonds the flexible printed board 18 to the light introduction part 16. Second leakage light is light L2 that leaks from the inclined surface 16a of the light introduction part 16. That is, the light L2 is incident to the light introduction part 16 through the light incident end face 14, directly output to the outside through the inclined surface 16a, and incident to the end face of the optical sheet. Third leakage light is light L3, which is reflected downward by the inclined surface 16a, is further reflected by the reflecting sheet 25, leaks from the upper surface of the light guide plate body 17, and is incident to the end face or lower surface of the optical sheet.
The light L1, the light L2, and the light L3 are not the light, which is guided in the light guide plate body 17 and output from the effective emission region, namely, the controlled light. Therefore, the effective emission region is unevenly irradiated near the light source 15 with the light L1, the light L2, and the light L3 to degrade visual quality. For example, when light A in FIG. 2 leaks in front of the light source 15 through an edge of the aperture window 24, eyespot luminescence P emerges at the edge of the effective emission region as illustrated in FIG. 3, thereby losing evenness of luminance in the effective emission region. For example, light B in FIG. 2 is incident to the optical sheet through the end face of the optical sheet, and leaks gradually while reflected in the optical sheet. Therefore, a bright line Q is generated in front of the light source 15 as illustrated in FIG. 4 to lose the evenness of the luminance in the effective emission region.
As illustrated in FIG. 5, there is a method for extending the diffusion sheet 20 onto the side of the light introduction part 16 (for example, see Japanese Unexamined Patent Publication No. 2006-93015). When the diffusion sheet 20 is extended onto the side of the light introduction part 16 to cover the inclined surface 16a with the diffusion sheet 20, the light L2 leaking from the inclined surface 16a is diffused by the diffusion sheet 20, so that the light leaking unevenly from a neighborhood of the light source 15 can be decreased. However, the method cannot solve the eyespot luminescence P in FIG. 3 caused by the leakage light.
As illustrated in FIG. 6, there is a method for extending the prism sheet 21 onto the side of the light introduction part 16 (for example, see Japanese Unexamined Patent Publication No. 2012-14909). According to the method, the light L2, which is output from the inclined surface 16a of the light introduction part 16 is incident to the optical sheet and leaks gradually from the optical sheet, can be absorbed by the rim sheet 23 to prevent the generation of the bright line Q. However, in the method, it is necessary that the rim sheet 23 have a sufficient length (a distance to the edge of the aperture window 24) DS, which is measured from the end of the prism sheet 21. An effect is not obtained when the distance DS is short. Therefore, the effective emission region of the area light source device is easy to narrow. Additionally, the method has no effect on the light L1 leaking directly from the light source 15, and the leakage light L1 leaks from the edge of the aperture window 24 to generate the eyespot luminescence P.